I have seen some quality Rustoleum paintjobs, honestly its all the prep work, taping and cut/buff that will give you the quality finish. Now a rattle can or Rustoleum paintjob won't look as great a year or two down the road but it can be done. If you want it to look good you do need to invest a bit of coin into a true automotive paint system.
First you need to pick a paint color and then find what manufacaturer makes it (custom mixes will be difficult if you ever need to touch up or respray a panel) , it will have it's own recommendations on how to apply it and usually paint companies also have their own primer and filler products that work directly with theirs. They are usually marketed as systems. You can mix and match products but you need to know what your doing in what order to see if the products label sheets work for your needs.
PPG, DuPoint etc all have their own systems. If your looking to save a little coin on the materials you can check out Eastwood in Alsip I have seen several of their products in finished stages and I can't tell the difference between their paint and a 5K paint system. I may use their products on my car when I get to that stage.
You need to examine the car you want to paint and determine if you need to strip it down to bare metal or if you are comfortable and trust that the existing paint will stay for the life of the paint job. In your example pic you see that the paint is coming away from the substrate, if that is the factory or aftermarket paintjob I highly recommend stripping the rest of the vehicle. It would really suck to do all that work and money spent to have the bottom layer start to bubble or peel away later down the road.
There are 2 types of adhesion, mechanical and chemical. Mechanical adhesion will be roughing up the substrate and then applying your base (epoxy primer on bare metal, sealer on painted surfaces) 220 grit is usually the recommendation but only in this step. Depending on how you apply the paint going forward you'll need different grit before you get to your color spray. This is where reading the label/data sheets, most sealers need to be scuffed if your not applying another layer within the 48 hours, if it's under the 48 hours you an tack the surface and apply the next coat (this would be the chemical adhesion).
Now for single stage or base +clear that depends on what you want the final product to look like. Both work, single stage is very durable and can be buffed to a very good finish and can last 40+ years. Base + clear will have a higher gloss and in some cases more depth to the paint. There are many arguments on which is easier or easier to correct but to keep this short pick the color you want, the paint system that works with that color and see what options you have and go from there.
As for jambs if you are doing a color change you should always do the jambs. It will stick out like a sore thumb if you don't. The jambs don't need as many layers as the outside of the car as their not exposed to the elements as much (UV exposure) and won't see the wear and tear. Even if your not doing a color change I still would do the jambs as there will be a big difference between the fresh paint and the older.
I know this was a longer post but this is just a small portion of the information. If you have questions on this or want more detail shoot me a PM.